Field of the Invention
The invention relates to a configuration for evaluating a signal that is read from a ferroelectric storage capacitor. The configuration has a read amplifier for amplifying the read signal, and an evaluating logic that is connected in series with the read amplifier.
As is generally known, owing to the characteristic hysteresis curve of the ferroelectric dielectric material of the storage capacitor, ferroelectric memories are non-volatile memories which are able to keep information long-term without the application of a supply voltage. In such ferroelectric memories, the information is stored in the form of a remanent polarization state of the ferroelectric dielectric material. Presently it is possible to store two states in this fashion, which are represented by a positive and a negative charge, respectively, and to which a xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d is allocated. Memory cells of such a memory are read by performing a comparison to a reference cell, whose state must be known. For instance, in memory cells formed of two transistors and two capacitors (2T2C cells), the reference cell polarization is always the opposite of the memory cell polarization.
The read amplifier that is connected in series with the ferroelectric memory works as a comparator that compares the charge quantity accepted in a memory cell being read to the charge quantity of the reference cell given the application of a read impulse. The cell whose polarization must be switched accepts the larger charge.
Taking the example of the 2T2C cell, if the memory cell carries a positive polarization, its reference cell has a negative polarization. If the memory cell is then read with a positive read impulse, the polarization of the reference cell inverts, whereas the state of the memory cell remains unchanged. Thus, in this case the reference cell accepts the larger charge compared to the memory cell. The resulting charge difference is then evaluated by the read amplifier.
If individual cells in a ferroelectric memory have insufficient data storage characteristics, or if a manipulation attempt is made, for instance by local or global temperature elevation, the dielectric materials of the affected cells lose part of their polarization, so that the cells tend to shift from both positive and negative polarization states to a xe2x80x9czeroxe2x80x9d polarization, or what is known as a zero state. If a memory cell and its reference cell are now in the zero state owing to a disturbance or a manipulation attempt, there is no longer an evaluatable polarization difference, and the signal at the output of the read amplifier is no longer predictably dependent upon factors such as tolerances which have nothing to do with the memory contents.
It is accordingly an object of the invention to provide a Configuration for evaluating a signal which is read from a ferroelectric storage capacitor which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the data storage in the individual cells is monitored, and an attempted data manipulation in the form of a local or global thermal attack can be immediately detected.
With the foregoing and other objects in view there is provided, in accordance with the invention, a configuration for evaluating a signal that is read from a ferroelectric storage capacitor. The configuration contains a read amplifier for amplifying the signal read from the ferroelectric storage capacitor, an evaluation unit connected in series with the read amplifier, and a counter connected between the read amplifier and the evaluation unit. The counter receives and is charged by a clock signal. After the signal has been amplified by the read amplifier, the counter converts a pulse width of the signal into a corresponding count, and, from a level of the count, the evaluation unit determines whether the signal was read from a high positive polarization region, a high negative polarization region, or a weak polarization region of a storage medium of the ferroelectric storage capacitor.
The object is inventively achieved in the configuration of the above-mentioned type in that a counter, which is charged with a clock signal, is provided between the read amplifier and the evaluating logic. The counter converts the pulse width of the read signal, upon its amplification by the read amplifier, into a corresponding count value. The evaluating logic determines, from the level of the count value, whether the signal was read from a region of high positive polarization, high negative polarization, or weak polarization of the storage medium of the ferroelectric storage capacitor.
Thus, a xe2x80x9cthird evaluation statexe2x80x9d (i.e. a weak polarization region) is introduced in the inventive configuration, which is situated on either side of the xe2x80x9czero statexe2x80x9d and which can be excluded from evaluation. If the polarization state of the cell being read is within the third evaluation state, the read amplifier can deliver a signal signaling the invalidity of the output data without further ado. By evaluating additional circumstances by monitor cells, for example, the evaluation logic can then decide, without further ado, whether there is a data storage problem or a manipulation attempt when the third evaluation state is detected in the read amplifier.
The width of the third evaluation state is expediently set to be symmetrical relative to the xe2x80x9czero statexe2x80x9d. The read amplifier has an output for signaling the state.
It is advantageous when the width of the third evaluation state is adjustable, specifically by the read amplifier. It is then possible to optimally adapt a memory to the requirements under which it operates. A large width of the third evaluation state is advantageous above all when there is a danger of manipulation attempts, whereas a smaller width is sufficient for applications in which there is no such danger of manipulation attempts.
Lastly, according to the invention it is possible to realize a memory cell that can store 1.5 bits in a storage capacitor, because the memory states can be expressed by positive polarization, negative polarization and no polarization, so that the xe2x80x9czero statexe2x80x9d can be utilized for storing a third information state. In any case, this requires a suitable circuit with which a cell can be put into a xe2x80x9czero statexe2x80x9d. This is possible by a depolarization circuit, in which the amplitude of a sinusoidal AC voltage at the cell is moved continuously downward.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a configuration for evaluating a signal which is read from a ferroelectric storage capacitor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.